1. Field of the Invention
This invention relates to the microscopic art and more particularly, to an apparatus and method for pretreating biological materials in the way suitable for observation by scanning electron microscopes.
2. Description of the Prior Art
As is well known, for observation of biological materials or specimens through scanning electron microscopes, primary electrons are irradiated on the surface of the specimen to generate secondary electrons therefrom having an energy below 100 eV and the thus generated secondary electrons are used as an information source of the image. However, because the biological specimen is an electrical insulator and tends to be electrically charged upon irradiation of the primary electrons, the resulting image may be defective and the secondary electrons may not be sufficiently emitted. This does not lead to high resolution for the material.
To avoid this, a biological specimen is coated, after fixation, dehydration and drying, by vacuum deposition with a metal such as gold onto the surface of the specimen. By this, emission of secondary electrons is ensured with the specimen being prevented from being electrically charged, thereby attaining high resolution. Where, however, a metal layer of several hundred angstroms in thickness is deposited on the specimen surface, it is difficult to observe fine portions of the specimen because of the presence of the metal layer. In addition, when gold or platinum is used for the vacuum deposition, the coating cost becomes high. With tungsten, the emissivity of secondary electrons is high with attendant high resolution but a problem is involved in that because of the high melting point of tungsten, a biological specimen which has poor resistance to heat tends to deteriorate by application of heat upon vacuum deposition.